If you have broken everything by renaming, editing or removing the default theme and you have trouble signing in to administration panel then in this case you can fix the problem by changing Drupal’s default theme in MySQL database. Open PHPMyAdmin or MySQL Workbench and execute the following sql queries.
Set Garland theme as default:
UPDATE system SET STATUS=1 WHERE name = 'garland';
Change the variable theme_default value:
UPDATE variable SET VALUE='s:7:"garland"' WHERE name = 'theme_default';